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With the development of nuclear weapons during and after World War Two the world entered a new era in which peace among the developed countries became maintained by the concept of Mutually Assured Destruction (or M.A.D.). This concept states that if any country were to attack another with nuclear weapons that they would trigger a global nuclear war that would lead to the deaths of millions around the globe. This sparked an arms race known as the Cold War as many developed countries quickly acted to prevent from being left vulnerable to external attack. The United States and the Union of Soviet Socialist Republics became the largest holders of nuclear weapons throughout the 20th century to even now. Citizens of both countries during the Cold War implored their governments to halt the arms race and to reduce the capacity for producing these weapons using similar rhetoric.